Dialogflow comes in 2 editions
Standard Edition is free and covers the need of most developers, while Enterprise Edition offers paid enterprise support. You can choose which edition to use when you create your Dialogflow agent. Your account can include agents from both editions.
See which edition works best for your development needs:
* Features labelled as "Unlimited" have default enterprise-ready quotas and limits. Dialogflow Enterprise Edition users may request quota increases from the GCP Console. Quota increases for usage in the context of Actions on Google (Google Assistant) will be provided free of charge for high quality actions upon request.
See the Google Cloud quota and limits page for more information.